Millimeter wave (mmWave) wideband channels in a multiple-input multiple-output (MIMO) transmission are described by a sparse set of impulse responses in the angle-delay, or space-time (ST), domain. These characteristics will be even more prominent in the THz band used in future systems. We consider two approaches for channel estimation: compressedsensing (CS), exploiting the sparsity in the angular/delay domain, and low-rank (LR), exploiting the algebraic structure of channel matrix. Both approaches share several commonalities, and this paper provides for the first time i) a comparison of the two approaches, and ii) new versions of CS and LR methods that significantly improve performance in terms of mean squared error (MSE), computational complexity, and latency. We derive the asymptotic MSE bound for any estimator of the ST-MIMO multipath channels with invariant angles/delays and time-varying fading, with unknown angle/delay diversity order: the bound also accounts for the degradation introduced by sub-optimal separable channel models. We will show that in the considered scenarios both CS and LR approaches attain the bound. Our performance assessment over ideal and 3 rd generation partnership project (3GPP) channel models, suitable for the fifth-generation (5G) and beyond of cellular networks, shows the trade-off obtained by the methods over various metrics: i) CS methods are converging faster than the LR methods, both attaining the asymptotic MSE bound; ii) the CS methods depend on the array manifold, while LR methods are independent of the array calibration; iii) CS solutions are more complex than LR solutions.

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1 In-region location verification (IRLV) aims at verifying whether a user is inside a region of interest (ROI). In wireless networks, IRLV can exploit the features of the channel between the user and a set of trusted access points. In practice, the channel feature statistics is not available and we resort to machine learning (ML) solutions for IRLV. We first show that solutions based on either neural networks (NNs) or support vector machines (SVMs) and typical loss functions are Neyman-Pearson (N-P)-optimal at learning convergence for sufficiently complex learning machines and large training datasets . Indeed, for finite training, ML solutions are more accurate than the N-P test based on estimated channel statistics.Then, as estimating channel features outside the ROI may be difficult, we consider one-class classifiers, namely auto-encoders NNs and one-class SVMs, which however are not equivalent to the generalized likelihood ratio test (GLRT), typically replacing the N-P test in the one-class problem. Numerical results support the results in realistic wireless networks, with channel models including path-loss, shadowing, and fading. Index TermsAuto-encoder, in-region location verification, machine learning, neural network, support vector machine.

The Fifth Generation of Communication Networks (5G) envisions a broader range of services compared to previous generations, supporting an increased number of use cases and applications. The broader application domain leads to increase in consumer use and, in turn, increased hacker activity. Due to this chain of events, strong and efficient security measures are required to create a secure and trusted environment for users. In this paper, we provide an objective overview of 5G security issues and the existing and newly proposed technologies designed to secure the 5G environment. We categorize security technologies using Open Systems Interconnection (OSI) layers and, for each layer, we discuss vulnerabilities, threats, security solutions, challenges, gaps and open research issues. While we discuss all seven OSI layers, the most interesting findings are in layer one, the physical layer. In fact, compared to other layers, the physical layer between the base stations and users' device presents increased opportunities for attacks such as eavesdropping and data fabrication. However, no single OSI layer can stand on its own to provide proper security. All layers in the 5G must work together, providing their own unique technology in an effort to ensure security and integrity for 5G data.

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